This invention relates to a method and apparatus for generating two clock signals phase locked to a common system clock but having different word rates.
Computers of various sizes are readily available at reasonable prices as a result of technological advances in micro electronics, i.e., integrated circuits. Computing power of digital computers has intensively been utilized for handling both graphical and alphanumerical information. To enhance human/computer interface, computer display terminals, including those employing high resolution color CRTs (cathode-ray tubes), as well as other types, become increasingly important in both office and laboratory environments.
It is preferable that such computer display terminals have the capability to display both graphic and alphanumerical information either on the entire CRT screen or selected areas thereof, with a selectable relationship therebetween, which may include a superimposed display of alphanumerics over a background graphic display, or vice versa. A raster scan type computer terminal display system architecture which utilizes separate subsystems for graphic and alphanumerics (text or dialog) requires two different timing generators operating from a common pixel rate system clock, but generating word rate clocks of different lengths. In this application, the two clocks not only have to have a fixed phase with each other but also a fixed phase with the horizontal period of the master CRT controller so that the alphanumerics display will operate correctly. In prior art systems, separate clock generators are dependent upon one another, resulting in difficult synchronizing problems. Moreover, in such prior art systems, the clock generators were synchronized once at initialization, offering no immunity from noise which might advance one clock generator, but not the other.